In cellular networks, it has been estimated that ⅔ of the voice calls and over 90% of data service utilization is performed indoors. However, in an indoor environment operators typically find it very difficult to provide good coverage and sufficient capacity, mainly due to the unfavorable indoor propagation conditions. Densification of macro base stations, e.g. LTE enhanced NodeBs (eNBs) that can overcome the indoor penetration loss may solve the problem to some extent. However, this is very expensive and sooner or later interference between the high power base stations is going to make optimizing and running such a dense macro network very challenging. Moreover, wireless terminals, e.g. user equipments (UEs), still suffer from battery drain as they also have to compensate for the indoor penetration loss in the uplink (UL) direction. Low power nodes (LPN), which can either be stand alone pico base stations (simply known as picos) or home base stations (HeNBs or HNBs, also known as femto cell base stations or simply femtos) is one of the solutions proposed for solving the indoor coverage and capacity problem. A pico base station is the same as a macro eNB apart from its smaller coverage area, while HeNB/HNBs have some architectural differences from macro eNB.
“Low power node” in the present context refers to a node having a lower maximum output power compared to a macro base station. Due to their comparatively smaller coverage area, low power nodes may be deployed within the macro cells as an additional layer, providing “hot spots” of additional coverage where needed. The terms “local area base station” or “local area node” may be used to refer to an LPN, in particular a pico base station. Home base stations may also be viewed as a special type of local area node. In contrast, a macro base station may be referred to as a “wide area” base station.
3GPP TS 36.104, version 10.5.0, table 6.2-1, which is included below, provides example output power levels for different types of base stations. However, it should be realized that the exact output power may vary, and the examples should thus be viewed as non-limiting.
TABLE 6.2-1Base Station rated output powerBS classPRATWide Area BS- (note)Local Area BS≦+24 dBm (for one transmitantenna port)≦+21 dBm (for two transmitantenna ports)≦+18 dBm (for four transmitantenna ports)<+15 dBm (for eight transmitantenna ports)Home BS≦+20 dBm (for one transmitantenna port)≦+17 dBm (for two transmitantenna ports)≦+14 dBm (for four transmitantenna ports)<+11 dBm (for eight transmitantenna ports)NOTE:There is no upper limit for the rated output power of the Wide Area Base Station.
Some of the benefits that LPNs are expected to offer are:                Offload of traffic from macro eNBs, and hence an increase in macro layer capacity.        Guaranteed coverage and high capacity transmission at home.        UE battery savings due to low UL transmission power requirements.        Seamless connectivity when UEs move in and out of homes, apartment and office buildings, etc. Although cellular to WiFi handover is possible, all currently existing solutions are too slow and complex to realize seamless handover.        
Femto cells have been defined in the 3rd Generation Partnership Project (3GPP) Release 8, for both Universal Mobile Telecommunication System (UMTS) and Evolved Packet System (EPS) technologies. EPS comprises the Long Term Evolution (LTE). UMTS femtos are referred to as HNBs while EPS/LTE femtos are called HeNBs. Throughout this disclosure, the notation H(e)NB is used to denote both HNBs and HeNBs.
A HeNB is an LPN, typically located at a residential home or office, which can provide connectivity to cellular users over the Internet, for example, using the user's dedicated Digital Subscriber Line (DSL). HeNB are generally physically small, with similar dimensions as a WiFi access point. FIG. 1 shows a typical deployment scenario of HeNBs, where eNBs are providing coverage mainly for outdoor UEs and the HeNBs offload some of the indoor traffic. Since H(e)NBs are low cost base stations that may be user deployed, it is important to have a flexible access control mechanism, considering both performance and security aspects. Currently, three access modes are defined in 3GPP:                Closed access: Only a subset of UEs are allowed to connect to the H(e)NB. This access mode is also known as Closed Subscriber Group (CSG).        Open access: This access mode is similar to a normal eNB access mode, i.e. all customers of the operator are allowed to use the H(e)NB.        Hybrid access: This is a mixture of CSG and open access, where any user is allowed to connect to the H(e)NB, but the UEs that are members of the CSG of the H(e)NB might get priority or different charging rates as compared with non-CSG UEs.        
On the other hand, picos are always expected to be in open access mode like macro eNBs.
Throughout this disclosure, the expression “CSG cell” refers to a cell being served by a base station in closed access mode. The base station is typically a H(e)NB, however it is not excluded that other types of base stations could operate in closed access mode and serve CSG cells.
In EPS/LTE, HeNBs broadcast their access mode in the System Information Block Type 1 (SIB1), using the CSG-Indication and CSG ID parameters. CSG-Indication is set to TRUE only in the case of closed HeNBs, and CSG ID is only present in closed and hybrid HeNBs. A list of CSG IDs that the user has access to is stored in the UE in a list called CSG white list, i.e. UEs can not access closed HeNB cells that have a CSG ID that is not included in the UE's CSG white list. When accessing a hybrid access HeNB, the presence or absence of the CSG ID in the CSG white list determines whether the UE is given preferential treatment, such as higher priority and/or lower charging rates, or not.
Only outbound handover, i.e. HeNB-to-eNB, was supported in Release 8, but starting with Release 9, inbound handover from eNB to HeNB is also supported. Performing this procedure in the same way as an all-macro deployment can be very expensive in terms of the time required for the UE to perform neighbor cell measurements and also the overhead of measurement reports. This is because, due to the anticipated dense deployment of the HeNBs, a UE might be able to detect a large number of HeNB cells. Some of these, often even a large majority of them, might not be relevant to the UE if they are CSG cells in which the UE has no membership.
Thus, the concept of proximity reporting was introduced in release 9, where the UE can indicate to the serving eNB whenever it is entering or leaving the proximity of cells with CSG IDs that the UE has in its white list. The proximity detection, also known as Autonomous Search Function (ASF), is not standardized and is left for UE implementation. ASF can be based, for example, on location information, e.g. GPS location indicating that the UE is approaching home, or some other kind of fingerprinting where the UE maps the location based on the Physical Cell Identity (PCI) of the neighboring cells of its HeNB. The fingerprinting procedure, whether it is based on location or neighbor cell information, may be configured (i.e. the location or neighbor cell PCIs are learnt) the first time the user connects to a HeNB and may also be refined (e.g. adding and/or removing neighbor cell PCIs), or verified, at subsequent occasions when the UE connects to the HeNB.
The UE sends the proximity indication later on to its serving macro cell eNB whenever the current location or neighbor cell PCIs match(es) the fingerprint. The UEs can be disabled from measuring the CSG cells until they detect that they are nearby an allowed CSG cell and send a proximity indication. This can be done by configuring them not to measure on the frequency used by the HeNBs in the case of inter-frequency deployment. In the case of intra-frequency deployment, the UEs can be configured to put the PCIs used by CSG cells in their black cell list. A black cell list consists of cells which are not to be considered as potential handover target cells and which the UE consequently should not include in measurements and measurement reports.
When the serving cell receives a proximity indication from a UE, it can re-enable UE measurements of the CSG cells. For example, the serving cell may enable measurements at the HeNB frequency or put the PCIs of the CSG cells in the white list.
The procedure for inbound mobility towards a closed/hybrid HeNB is illustrated in FIG. 2 (see also 3GPP TS 36.300, version 10.6.0, section 10.5.1). The following five basic steps can be identified:                A. Proximity configuration/reporting: The source eNB configures the UE whether to send proximity indication reports or not, and a UE configured to report proximity will do so accordingly whenever it detects that it is approaching a HeNB whose CSG ID is in its CSG white list. To reduce the number of reports, a UE is limited to send not more than one proximity indication within 5 sec (see 3GPP TS 36.331, version 10.5.0).        B. Handover measurement/reporting: The source eNB configures the UE with relevant measurement configuration (such as measurement gaps), if such configuration is not already present. The UE includes the PCI in the measurement reports.        C. System Information acquisition: The source eNB configures the UE to perform System Information (SI) acquisition and reporting for the cell with the concerned reported PCI. The UE sends the requested measurement, which includes information such as the E-UTRAN Cell Global Identifier (CGI) (which uniquely identifies a cell as opposed to the PCI that can be reused by other cells), the CSG ID and “member/non-member” indication.        D. Access control: The Mobility Management Entity (MME) checks if the UE is allowed to access the reported CSG cell, and the target HeNB checks if the reported CSG ID is the same as the CSG ID that it is broadcasting.        E. Handover preparation and execution: This is done in parallel with the previous step (D), where the MME forwards the HO required message towards the target HeNB and the target HeNB responds with HO request acknowledged message. The source eNB, upon getting the acknowledgement, will order the UE to execute HO towards the target HeNB.        
Most measurements in LTE are done by the UE on the serving cell as well as on neighbor cells over some known Reference Symbols (RS) or pilot sequences. The measurements are done for various purposes. Some example measurement purposes are: mobility, positioning, self organizing network (SON), minimization of drive tests (MDT), operation and maintenance (O&M), network planning and optimization etc. The measurements may also comprise cell identification e.g. acquisition of the PCI, the CGI/ECGI, CSG-ID, and/or the System Information of the target cell, be it an LTE cell or any inter-RAT cell.
Examples of mobility measurements in LTE are:                Reference Symbol Received Power (RSRP)        Reference Symbol Received Quality (RSRQ)        
Examples of well known positioning measurements in LTE are:                Reference Signal Time Difference (RSTD)        RX-TX time difference measurement        
Some measurements may also require the eNB to measure the signals transmitted by the UE in the uplink. One important measurement performed by the eNB in LTE is the estimation of Timing Advance (TA). For LTE, uplink orthogonality is required to avoid intra-cell interference and as such it is important to have all the uplink signals time-aligned when they are received at the eNB. Thus, eNBs try to compensate for the propagation delay differences of their UEs (due to their differing distances from the eNB), by instructing them to apply different timing advances, and the UEs will apply the configured timing advance when they are transmitting. The TA can first be estimated during the initial random access procedure when the UE establishes a connection with the eNB (either due to handover or going from idle to connected mode). TA updates are then performed throughout the duration the UE is connected to the eNB, as the propagation delay might change, for example due to the movement of the UE, the change of the environment due to movement of other objects in a dense urban setting, etc. For these updates, the eNBs may measure received uplink signals such as Sounding Reference Signals (SRS), Channel Quality Indicator (CQI), ACKs and NACKs in response to downlink data reception, or the uplink data transmission. The details of uplink timing measurements at the eNB are not standardized and left to implementation.
eNBs that have multiple antenna elements could also use their diversity to measure the Angle of Arrival (AoA) of the uplink signals that they receive from their UEs. The AoA and TA can be used to estimate the relative coordinates of the UEs within the cell.
The PCI is an essential configuration parameter of a radio cell. PCIs are grouped into 168 unique physical layer cell identity groups, each group containing 3 unique identities. Thus, there are only 504 different PCIs altogether (see 3GPP TS 36.211, version 10.4.0). Limiting the number of PCIs makes the initial PCI detection by the UE during cell search easier, but the limited number of PCIs inevitably leads to the reuse of the same PCI values in different cells. Therefore, a PCI might not uniquely identify a neighbor cell, and each cell additionally broadcasts, as a part of the system information (SI), a globally unique cell identifier (CGI/ECGI).
When a new node (e.g. an eNB or HeNB) is brought into the field, a PCI needs to be selected for each of its supported cells, avoiding collision with respective neighboring cells. The use of identical PCI by two cells in close proximity results in interference conditions that might hinder the identification and use of any of them. Otherwise if both cells have a common neighbor, handover measurements that are based on PCI will become ambiguous thus leading to confusing measurement reports or even to the handing over of a UE to the wrong cell, which can cause Radio Link Failure (RLF).
The PCI assignment shall fulfill the following two conditions:                Collision-free: The PCI is unique in the area that the cell covers        Confusion-free: a cell shall not have more than one neighboring cell with identical PCI        
Using an identical PCI for two cells creates collision, which can only be solved by restarting at least one of the cells and reassigning PCIs upon restart, causing service interruption. PCI confusion, on the other hand, can be resolved by instructing the UEs to read the CGI of the concerned neighbor cell. However, this might require the UEs to stop transmitting/receiving from their serving node during the idle period that is required to read the neighbor's system information, which can be in the range of 250 ms. Therefore, putting a PCI in use which causes either collision or confusion is highly undesirable.
Traditionally, a proper PCI is derived from radio network planning and is part of the initial configuration of the node. The network planning tool calculates the possible PCIs for the new cell(s) based on estimated neighbor relations of the new cells, as estimated by cell coverage area predictions. However, prediction errors, due to imperfections in map and building data, and to inaccuracies in propagation models, have forced operators to resort to drive/walk tests to ensure proper knowledge of the coverage region and identify all relevant neighbors and handover regions. Even the accuracy of that is questionable as some factors such as seasonal changes (the falling of leaves or snow melting) can alter the propagation conditions. Also, the inaccuracy of cell coverage and neighbor relation assessment increases with time as the live network and its surroundings evolve over time.
LTE has support for a feature known as UE ANR (User Equipment Automatic Neighbor Relations), which allows UEs to decode and report the CGI/ECGI information of neighbor cells (in addition to the CSG cell ID in the case of HeNBs) to the serving cell upon request. eNBs maintain a neighbor relation table (NRT) for each of their cells. Apart from the PCI to CGI/ECGI mapping, each neighbor relation contains other relevant information such as the possibility of X2 connectivity.
The CGIs/ECGIs of the neighbor cells are the ones that are used when signaling to the neighbor eNB via the MME, since the MME routes the messages based on eNB identity which is a part of CGI/ECGI. If the policy is to establish X2 for neighbor relations and if X2 is not already available, then the CGI/ECGI can be used to retrieve the target node's IP address, which is used for X2 setup. When the X2 interface is established, the neighboring eNBs can share information about their served cells including PCIs and CGIs/ECGIs. It is also possible to share such information via the Operation and Maintenance (OAM or O&M) system.
As explained above, fingerprinting and proximity indication by user equipments may help reduce the number of neighbor cell measurements and prevent the transmission of unnecessary measurement reports, in particular in heterogeneous network scenarios.
However, the existing mechanisms discussed above are insufficient and/or unreliable in some situations. Thus, there is a need for further improvements in this area.